Semiconductor laser devices with a tunable wavelength are increasingly important as light sources for advanced lightwave communications systems, for example, for wavelength division multiplexed (WDM) systems. A type of a tunable laser diode is a three-section distributed Bragg reflector (DBR) laser, which is schematically illustrated in FIG. 1. As shown in FIG. 1, a DBR laser 10 includes a tuning region 12 with a tuning waveguide 13a and grating 13b and, possibly, a facet coating 16. The DBR laser 10 further includes a phase-tuning section 12b and a phase-tuning waveguide 13c. A gain section 14 with an active waveguide 14a, typically containing a multi-quantum well (MQW) stack, generates spontaneous emission and optical gain about an energy determined by the quantum well structure. With application of electrical current to electrodes 17 and 18, a threshold is reached beyond which sufficient gain and optical feedback from mirror 19 and the distributed Bragg reflector formed by 13a and 13b promotes laser action.
Optimization of lasing action in tunable sources, such as the DBR laser 10 of FIG. 1, requires increased tuning efficiency, narrow linewidth and immunity to noise. Tuning efficiency is defined as the rate of frequency change for a given change of tuning current to a tunable laser. The tuning range, which is defined as the range of wavelengths over which the laser can tune for a given amount of tuning current, is defined as the integral of the tuning efficiency over the range of tuning current. The tuning range may be quantified as a function of the number of tuning steps over which a laser device lases. For example, FIG. 2 illustrates the output wavelength of light from a laser device as a function of the current injected in the tuning region 12 of the laser. As shown diagrammatically in FIG. 2, over a tuning current range of about 0.1 mA to about 20 mA, the laser can tune in eight (8) tuning steps over about 4 nm wavelength tuning range (the difference between 1551 nm and 1547 nm).
Optimization of the tuning range requires a large or broad tuning range of the laser device because, with a large tuning range, a tunable laser can produce a wide variety of output frequencies. As known in the art, broad tuning range is conferred by a high sensitivity of the effective refractive index of the tuning section to variations in the tuning current. Nevertheless, the same high sensitivity of refractive index that provides a large tuning range also renders a tunable laser susceptible to shot noise, current noise and external optical reflection noise.
The change of refractive index of the tuning section is substantially linear with the density of carriers injected into the waveguide layer of tuning region 12. In turn, the carrier density is related to the injection current by the recombination equation (1):I/qV=An+Bn2+Cn3  (1)where,
I=the intensity of injected current;
q=the electron charge;
V=the volume of the tuning waveguide layer;
A=linear coefficient;
B=quadratic coefficient;
C=cubic coefficient; and
n=the density of the injected carriers.
The linear coefficient A of equation (1) is associated with monomolecular recombination, which in turn is due to non-radiative recombination through traps, damage or defects in the material, for example, and produces a dominant effect at low current densities. The quadratic coefficient B is associated with radiative bimolecular recombination, which produces spontaneous emission of light, while the cubic coefficient C is associated with non-radiative Auger recombination. Radiative and Auger recombination dominate over monomolecular recombination at high carrier densities.
Recently, efforts have been made at optimizing the tuning efficiency and the tuning range in tunable lasers and at minimizing the noise, for example, the shot noise and the current supply noise. According to equation (1), the change of carrier density with current, and therefore the tuning efficiency, is greatest at low carrier density and decreases with increasing current. Recent studies have proposed heavy doping of the waveguide layer of tuning region 12 to increase the recombination rate of carriers. This method increases the background carrier density in the waveguide layer to a high enough level so that radiative and Auger are the dominant recombination mechanisms, thus avoiding the region of high tuning efficiency at low carrier density. Nevertheless, because the carrier density is already high at zero current, this approach dramatically reduces the range over which the carrier density may be changed, resulting in a dramatic reduction in tuning range.
Accordingly, there is a need for an improved method for increasing the immunity to noise of a tunable device without compromising the tuning range. There is also a need for a tunable laser that can be easily integrated with WDM components and that has increased immunity to noise as well as large tuning range.